Loop yoke for proprotor systems

ABSTRACT

A yoke for providing a centrifugal force retention load path between a proprotor blade and a hub of a soft-in-plane proprotor system operable for use on a tiltrotor aircraft. The yoke includes a continuous loop having a longitudinal axis and first and second longitudinal sections extending between inboard and outboard arcuate sections. A flapping bearing receiving region is disposed at least partially within the inboard arcuate section to an interior of the continuous loop. A centrifugal force bearing receiving region is disposed at least partially within the outboard arcuate section to the interior of the continuous loop. The continuous loop is formed from a composite material having a plurality of double bias material plies and a plurality of unidirectional material plies such that the number of unidirectional material plies is greater than the number of double bias material plies.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to rotor systems operablefor use on rotorcraft and, in particular, to soft-in-plane proprotorsystems including a hub and a plurality of proprotor blades operable foruse on tiltrotor aircraft.

BACKGROUND

Tiltrotor aircraft typically include multiple propulsion assemblies thatare positioned near outboard ends of a fixed wing. Each propulsionassembly may include an engine and transmission that provide torque androtational energy to a drive shaft that rotates a proprotor systemsincluding a hub and a plurality of proprotor blades. Typically, at leasta portion of each propulsion assembly is rotatable relative to the fixedwing such that the proprotor blades have a generally horizontal plane ofrotation providing vertical thrust for takeoff, hovering and landing,much like a conventional helicopter, and a generally vertical plane ofrotation providing forward thrust for cruising in forward flight withthe fixed wing providing lift, much like a conventional propeller drivenairplane. In addition, tiltrotor aircraft can be operated inconfigurations between the helicopter mode and the airplane mode, whichmay be referred to as conversion mode.

Physical structures have natural frequencies of vibration that can beexcited by forces applied thereto as a result of operating parametersand environmental conditions. These frequencies are determined, at leastin part, by the materials and geometrical dimensions of the structures.In the case of tiltrotor aircraft, certain structures having criticalnatural frequencies include the fuselage, the fixed wing and variouselements of the propulsion assemblies. One important operating parameterof a tiltrotor aircraft is the angular velocity or revolutions perminute (RPM) of the proprotor blades, which may generate excitationfrequencies corresponding to 1/rev (1 per revolution), 2/rev, 3/rev,etc. An important environmental condition experienced by tiltrotoraircraft is forward airspeed, which may induce proprotor aeroelasticinstability, similar to propeller whirl flutter, that may couple to thefixed wing of tiltrotor aircraft. It has been found that forwardairspeed induced proprotor aeroelastic instability is a limiting factorrelating to the maximum airspeed of tiltrotor aircraft in airplane mode.

SUMMARY

In a first aspect, the present disclosure is directed to a proprotorsystem for tiltrotor aircraft having a helicopter mode and an airplanemode. The proprotor system includes a hub and a plurality of proprotorblades coupled to the hub such that each proprotor blade is operable toindependently flap relative to the hub about a flapping axis andindependently change pitch about a pitch change axis wherein, a firstin-plane frequency of each proprotor blade is less than 1.0/rev.

In some embodiments, the first in-plane frequency of each proprotorblade may be between about 0.6/rev and about 0.9/rev. In one example,the first in-plane frequency of each proprotor blade in the helicoptermode may be between about 0.6/rev and about 0.7/rev and the firstin-plane frequency of each proprotor blade in the airplane mode may bebetween about 0.8/rev and about 0.9/rev. In certain embodiments, theproprotor system may include at least four proprotor blades or at leastfive proprotor blades. In some embodiments, a pitch control assemblyhaving a positive delta 3 angle may be operably coupled to eachproprotor blade. In such embodiments, the positive delta 3 angle may beup to about 35 degrees.

In certain embodiments, in the airplane mode, the proprotor system maybe operable between about 70 percent and about 80 percent of design RPM,the expected RPM in helicopter mode. In other embodiments, in theairplane mode, the proprotor system may be operable between about 60percent and about 70 percent of design RPM, between about 50 percent andabout 60 percent of design RPM and/or between about 40 percent and about50 percent of design RPM. In some embodiments, a lead-lag damper may bedisposed between each proprotor blade and the hub. In such embodiments,the lead-lag dampers may be elastomeric dampers or fluid dampers.

In a second aspect, the present disclosure is directed to a tiltrotoraircraft having a helicopter mode and an airplane mode. The tiltrotoraircraft includes a fuselage, a wing extending from the fuselage, atleast one drive system supported by at least one of the fuselage and thewing and at least one proprotor system coupled to the drive system. Theproprotor system includes a hub and a plurality of proprotor bladescoupled to the hub such that each proprotor blade is operable toindependently flap relative to the hub about a flapping axis andindependently change pitch about a pitch change axis wherein, a firstin-plane frequency of each proprotor blade is less than 1.0/rev.

In a third aspect, the present disclosure is directed to a soft-in-planeproprotor system operable for use on a tiltrotor aircraft having ahelicopter mode and an airplane mode. The proprotor system includes ahub and a plurality of proprotor blades coupled to the hub such thateach proprotor blade is operable to independently flap relative to thehub about a flapping axis and independently change pitch about a pitchchange axis. Each of a plurality of loop yokes couples one of theproprotor blades with the hub and includes first and second longitudinalsections extending between inboard and outboard arcuate sections. Abearing assembly is disposed between the inboard and outboard arcuatesections of each loop yoke. Each bearing assembly includes a flappingbearing disposed generally within the inboard arcuate section of therespective loop yoke and coupled to the hub, a lead-lag damper coupledto the hub, a centrifugal force bearing disposed generally within theoutboard arcuate section of the respective loop yoke and a blade anchorcoupled between the lead-lag damper and the centrifugal force bearing.The blade anchor is also coupled to the respective proprotor blade.

In some embodiments, the first in-plane frequency of each proprotorblade may be less than 1.0/rev. In such embodiments, the first in-planefrequency of each proprotor blade may be between about 0.6/rev and about0.9/rev. In one example, the first in-plane frequency of each proprotorblade in the helicopter mode may be between about 0.6/rev and about0.7/rev and the first in-plane frequency of each proprotor blade in theairplane mode may be between about 0.8/rev and about 0.9/rev. In certainembodiments, the proprotor system may include at least four proprotorblades or at least five proprotor blades. In some embodiments, a pitchcontrol assembly having a positive delta 3 angle may be operably coupledto each proprotor blade. In such embodiments, the positive delta 3 anglemay be up to about 35 degrees.

In certain embodiments, the loop yokes may be formed from a compositematerial having a plurality of double bias material plies and aplurality of unidirectional material plies with multiple unidirectionalmaterial plies interposed between the double bias material plies andwith a ratio of unidirectional material plies to double bias materialplies between about 2 to 1 and about 6 to 1. In some embodiments, theflapping bearings may be twin conical elastomeric flapping bearings, thelead-lag dampers may be elastomeric dampers or fluid dampers and mayhave a spring rate and/or the centrifugal force bearings may be twinspherical elastomeric bearings. In certain embodiments, each bearingassembly may include a bearing support disposed between the centrifugalforce bearing and the outboard arcuate section of the loop yoke.

In a fourth aspect, the present disclosure is directed to a tiltrotoraircraft having a helicopter mode and an airplane mode. The aircraftincludes a fuselage, a wing extending from the fuselage and at least onedrive system supported by the fuselage or the wing. At least onesoft-in-plane proprotor system is coupled to the drive system. Theproprotor system includes a hub and a plurality of proprotor bladescoupled to the hub such that each proprotor blade is operable toindependently flap relative to the hub about a flapping axis andindependently change pitch about a pitch change axis. Each of aplurality of loop yokes couples one of the proprotor blades with the huband includes first and second longitudinal sections extending betweeninboard and outboard arcuate sections. A bearing assembly is disposedbetween the inboard and outboard arcuate sections of each loop yoke.Each bearing assembly includes a flapping bearing disposed generallywithin the inboard arcuate section of the respective loop yoke andcoupled to the hub, a lead-lag damper coupled to the hub, a centrifugalforce bearing disposed generally within the outboard arcuate section ofthe respective loop yoke and a blade anchor coupled between the lead-lagdamper and the centrifugal force bearing. The blade anchor is alsocoupled to the respective proprotor blade.

In a fifth aspect, the present disclosure is directed to a yoke forproviding a centrifugal force retention load path between a proprotorblade and a hub of a soft-in-plane proprotor system operable for use ona tiltrotor aircraft. The yoke includes a continuous loop having alongitudinal axis and first and second longitudinal sections extendingbetween inboard and outboard arcuate sections. A flapping bearingreceiving region is disposed at least partially within the inboardarcuate section to an interior of the continuous loop. A centrifugalforce bearing receiving region is disposed at least partially within theoutboard arcuate section to the interior of the continuous loop. Thecontinuous loop is formed from a composite material having a pluralityof double bias material plies and a plurality of unidirectional materialplies wherein, the number of unidirectional material plies is greaterthan the number of double bias material plies.

In some embodiments, the composite material of the continuous loop mayinclude multiple unidirectional material plies interposed between thedouble bias material plies. In such embodiments, a preferred ratio ofunidirectional material plies to double bias material plies may bebetween about 2 to 1 and about 6 to 1, a more preferred ratio ofunidirectional material plies to double bias material plies may bebetween about 3 to 1 and about 5 to 1 and a most preferred ratio ofunidirectional material plies to double bias material plies may be about4 to 1. In certain embodiments, the double bias material plies of thecomposite material of the continuous loop may be double bias carbonfiber fabric such as double bias carbon fiber fabric with plus and minus45 degree orientation relative to the longitudinal axis of thecontinuous loop. In some embodiments, the unidirectional material pliesof the composite material of the continuous loop may be unidirectionalcarbon fiber fabric such as unidirectional carbon fiber fabric with 0degree orientation relative to the longitudinal axis of the continuousloop. In certain embodiments, the first and second longitudinal sectionsmay be generally parallel to one another. In some embodiments, the firstand second longitudinal sections may be tapered between the inboardarcuate section toward the outboard arcuate section.

In a sixth aspect, the present disclosure is directed to a yokemanufacturing method for yokes comprising a continuous loop having alongitudinal axis and first and second longitudinal sections extendingbetween inboard and outboard arcuate sections forming a flapping bearingreceiving region at least partially within the inboard arcuate sectionand a centrifugal force bearing receiving region at least partiallywithin the outboard arcuate section to the interior of the continuousloop, the yokes providing centrifugal force retention load paths betweenproprotor blades and a hub of a soft-in-plane proprotor system operablefor use on a tiltrotor aircraft. The method includes providing amandrel; laying up a plurality of double bias material plies and aplurality of unidirectional material plies on the mandrel in a sequenceincluding: (a) laying up a double bias material ply; (b) laying up aplurality of unidirectional material plies; (c) repeating steps (a) and(b) to achieve a predetermined thickness; and (d) laying up a doublebias material ply; curing the material plies with a resin to form acured yoke assembly; cutting the cured yoke assembly into a plurality ofyoke members; and finishing the yoke members to form the yokes.

In a seventh aspect, the present disclosure is directed to a tiltrotoraircraft. The aircraft includes a fuselage, a wing extending from thefuselage and at least one drive system supported by the fuselage or thewing. At least one soft-in-plane proprotor system is coupled to thedrive system. The proprotor system includes a plurality of proprotorblades each supported by a hub via a yoke. Each yoke includes acontinuous loop having a longitudinal axis and first and secondlongitudinal sections extending between inboard and outboard arcuatesections. A flapping bearing receiving region is disposed at leastpartially within the inboard arcuate section to an interior of thecontinuous loop. A centrifugal force bearing receiving region isdisposed at least partially within the outboard arcuate section to theinterior of the continuous loop. The continuous loop is formed from acomposite material having a plurality of double bias material plies anda plurality of unidirectional material plies wherein, the number ofunidirectional material plies is greater than the number of double biasmaterial plies.

In an eighth aspect, the present disclosure is directed to asoft-in-plane proprotor system operable for use on a tiltrotor aircrafthaving a helicopter mode and an airplane mode. The proprotor systemincludes a hub and a plurality of proprotor blades coupled to the hubsuch that each proprotor blade is operable to independently flaprelative to the hub about a flapping axis and independently change pitchabout a pitch change axis. A blade support assembly couples each of theproprotor blades with the hub. Each of the blade support assembliesincludes a flapping bearing coupled to the hub and a yoke having firstand second longitudinal sections with outboard grip members and aninboard arcuate section connecting the first and second longitudinalsections and coupled to the flapping bearing. A lead-lag damper iscoupled between the hub and an inboard station of the respectiveproprotor blade. A twist shank is coupled between the outboard gripmembers of the yoke and an outboard station of the respective theproprotor blade. The twist shank defines a virtual lead-lag hingeoutboard of the yoke and coincident with the respective pitch changeaxis.

In some embodiments, a first in-plane frequency of each proprotor blademay be less than 1.0/rev such as a first in-plane frequency of betweenabout 0.6/rev and about 0.9/rev. In such embodiments, the first in-planefrequency of each proprotor blade may be between about 0.6/rev and about0.7/rev in the helicopter mode and between about 0.8/rev and about0.9/rev in the airplane mode. In certain embodiments, each twist shankmay be operable to twist in a collective range between about plus 50degrees and about minus 50 degrees. Alternatively or additionally, eachtwist shank may have an in-plane spring rate. In some embodiments, eachtwist shank may be a beam having an inboard end, a central section andan outboard end wherein, the beam may have a generally tapered sectionfrom the inboard end toward the central section and from the outboardend toward the central section.

In certain embodiments, the virtual lead-lag hinge may be disposedbetween about a 15 percent station and about a 40 percent station of thetwist shank. In such embodiments the virtual lead-lag hinge may bedisposed between about a 20 percent station and about a 30 percentstation of the twist shank. In some embodiments, each twist shank may beformed from a plurality of material layers including a plurality of highstrength layers having resilient layers interposed therebetween such asa plurality of fiberglass layers having rubber layers interposedtherebetween. In certain embodiments, the proprotor system may includeat least four proprotor blades. In other embodiments, the proprotorsystem may include at least five proprotor blades. In some embodiments,a pitch control assembly having a positive delta 3 angle may be operablycoupled to each proprotor blade.

In a ninth aspect, the present disclosure is directed to a tiltrotoraircraft having a helicopter mode and an airplane mode. The tiltrotoraircraft includes a fuselage, a wing extending from the fuselage, atleast one drive system supported by at least one of the fuselage and thewing and at least one soft-in-plane proprotor system coupled to thedrive system. The proprotor system includes a hub, a plurality ofproprotor blades coupled to the hub such that each proprotor blade isoperable to independently flap relative to the hub about a flapping axisand independently change pitch about a pitch change axis and a pluralityof blade support assemblies, each coupling one of the proprotor bladeswith the hub. Each blade support assembly includes a flapping bearingcoupled to the hub and a yoke having first and second longitudinalsections with outboard grip members and an inboard arcuate sectionconnecting the first and second longitudinal sections and coupled to theflapping bearing. A lead-lag damper is coupled between the hub and aninboard station of the respective proprotor blade. A twist shank iscoupled between the outboard grip members of the yoke and an outboardstation of the respective the proprotor blade. The twist shank defines avirtual lead-lag hinge outboard of the yoke and coincident with therespective pitch change axis.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1B are schematic illustrations of a tiltrotor aircraft in anairplane mode and a helicopter mode, respectively, in accordance withembodiments of the present disclosure;

FIGS. 2A-2B are top views of a proprotor system for use on a tiltrotoraircraft in accordance with embodiments of the present disclosure;

FIGS. 2C-2D are isometric views of a proprotor system for use on atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 3 is an exploded view of a bearing assembly and a loop yoke of aproprotor system for use on a tiltrotor aircraft in accordance withembodiments of the present disclosure;

FIGS. 4A-4C are cross sectional views depicting an elastomeric damperduring in-plane oscillation of a proprotor blade of a proprotor systemfor use on a tiltrotor aircraft in accordance with embodiments of thepresent disclosure;

FIGS. 5A-5C are cross sectional views depicting a fluid damper duringin-plane oscillation of a proprotor blade of a proprotor system for useon a tiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIGS. 6A-6C are various views of a loop yoke of a proprotor system foruse on a tiltrotor aircraft in accordance with embodiments of thepresent disclosure;

FIGS. 7A-7E show processing steps for forming loop yokes for a proprotorsystem for use on a tiltrotor aircraft in accordance with embodiments ofthe present disclosure;

FIGS. 8A-8B are top views of a proprotor system for use on a tiltrotoraircraft in accordance with embodiments of the present disclosure;

FIGS. 8C-8D are isometric views of a proprotor system for use on atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 9 is an exploded view of a blade support assembly of a proprotorsystem for use on a tiltrotor aircraft in accordance with embodiments ofthe present disclosure;

FIGS. 10A-10C are various views of a twist shank of a proprotor systemfor use on a tiltrotor aircraft in accordance with embodiments of thepresent disclosure; and

FIGS. 11A-11C are cross sectional views depicting a blade supportassembly during in-plane oscillation of a proprotor blade of a proprotorsystem for use on a tiltrotor aircraft in accordance with embodiments ofthe present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction.

Referring to FIGS. 1A and 1B in the drawings, a tiltrotor aircraft isschematically illustrated and generally designated 10. Tiltrotoraircraft 10 includes a fuselage 12, landing gear 14, a tail member 16, awing 18 and housings 22, 24. Wing 18 is supported by fuselage 12 and maybe rotatable relative to fuselage 12 to place aircraft 10 in a storagemode wherein wing 18 is generally parallel with fuselage 12. In theillustrated embodiment, housings 22, 24 are fixedly attached in agenerally horizontal orientation to outboard ends of wing 18 and arenon-rotatable relative to wing 18. Mounted above wing 18 are pylons 26,28 that are at least partially rotatable relative to wing 18 andhousings 22, 24. Each pylon includes a proprotor system 30 having aplurality of proprotor blades 32. It should be understood by thosehaving ordinary skill in the art that even though the illustratedembodiment depicts proprotor systems having five proprotor blades, aproprotor system of the present disclosure could have alternate numbersof proprotor blades both greater than or less than five includingproprotor systems having three proprotor blades, proprotor systemshaving four proprotor blades or proprotor systems having at least sixproprotor blades. The position of pylons 26, 28, the angular velocity orrevolutions per minute (RPM) of the proprotor systems 30, the pitch ofproprotor blades 32 and the like are determined using a flight controlsystem, with or without pilot input, to selectively control thedirection, thrust and lift of tiltrotor aircraft 10 during flight.

It should be understood by those having ordinary skill in the art thatteachings of certain embodiments relating to the proprotor systems ofthe present disclosure described herein may apply to aircraft other thantiltrotor aircraft, such as non-tilting rotorcraft including helicopterrotor systems. In addition, it should be understood by those havingordinary skill in the art that teachings of certain embodiments relatingto the proprotor systems of the present disclosure described herein mayapply to aircraft other than rotorcraft, such as airplanes and unmannedaircraft, to name a few examples.

FIG. 1A illustrates tiltrotor aircraft 10 in a forward flight mode orairplane mode, in which proprotor systems 30 are positioned to rotate ina substantially vertical plane of rotation to provide a forward thrustwhile a lifting force is supplied by wing 18 such that tiltrotoraircraft 10 flies much like a conventional propeller driven aircraft.FIG. 1B illustrates tiltrotor aircraft 10 in a vertical takeoff andlanding flight mode or helicopter mode, in which proprotor systems 30are positioned to rotate in a substantially horizontal plane of rotationto provide a vertical thrust such that tiltrotor aircraft 10 flies muchlike a conventional helicopter. During flight operations, tiltrotoraircraft 10 may convert from helicopter mode to airplane mode followingvertical takeoff or hover and may convert back to helicopter mode fromairplane mode for hover or vertical landing. In addition, tiltrotoraircraft 10 can perform certain flight maneuvers with proprotor systems30 positioned between airplane mode and helicopter mode, which can bereferred to as conversion mode.

Preferably, each housing 22, 24 may be a nacelle having a drive system,such as an engine and transmission, disposed therein for supplyingtorque and rotational energy to a respective proprotor system 30. Insuch embodiments, the drive systems within each housing 22, 24 may becoupled together via one or more drive shafts located in wing 18 suchthat either drive system can serve as a backup to the other drive systemin the event of a failure. Alternatively or additionally, fuselage 12may include a drive system, such as an engine and transmission, forproviding torque and rotational energy to each proprotor system 30 viaone or more drive shafts located in wing 18. In tiltrotor aircrafthaving nacelle and fuselage mounted drive systems, the fuselage mounteddrive system may serve as a backup drive system in the event of failureof either or both of the nacelle mounted drive systems.

In general, proprotor systems for tiltrotor aircraft should be designedto achieve blade flap or out-of-plane frequencies and lead-lag orin-plane frequencies that are sufficiently distant from the excitationfrequencies generated by the proprotor systems corresponding to 1/rev (1per revolution), 2/rev, 3/rev, etc. As an example, if a proprotor systemhas an operating speed of 360 RPM, the corresponding 1/rev excitationfrequency is 6 Hertz (360/60=6 Hz). Similarly, the corresponding 2/revexcitation frequency is 12 Hz and the corresponding 3/rev excitationfrequency is 18 Hz. It should be understood by those having ordinaryskill in the art that a change in the operating speed of a proprotorsystem will result in a proportional change in the excitationfrequencies generated by the proprotor system. For tiltrotor aircraft,flight in airplane mode typically requires less thrust than flight inhelicopter mode. One way to reduce thrust as well as increase endurance,reduce noise levels and reduce fuel consumption is to reduce theoperating speed of the proprotor systems. For example, in helicoptermode, the tiltrotor aircraft may operate at 100 percent of design RPM,but in airplane mode, the tiltrotor aircraft may operate at a reducedpercent of design RPM such as between about 80 percent and about 90percent of design RPM, between about 70 percent and about 80 percent ofdesign RPM, between about 60 percent and about 70 percent of design RPM,between about 50 percent and about 60 percent of design RPM and/orbetween about 40 percent and about 50 percent of design RPM. Thus, toachieve desirable rotor dynamics, the proprotor systems for tiltrotoraircraft should be designed to avoid the frequencies of 1/rev, 2/rev,3/rev, etc. for both helicopter mode and airplane mode operations.

To achieve acceptable rotor dynamics, conventional tiltrotor aircrafthave operated proprotor systems having three twisted proprotor bladesutilizing negative 15 degrees delta 3 pitch-flap coupling and having afirst-in-plane frequency in airplane mode of about 1.4/rev. Delta 3refers to the angle measured about the rotational axis of the proprotorsystem from an axis normal to the pitch change axis to the pitch hornattachment point of a proprotor blade. Delta 3 pitch-flap coupling isused to reduce or control the degree of blade flapping by automaticallychanging the blade pitch as the blade flaps up or down relative to itsflap axis. It is noted that to achieve desired stability for aconventional helicopter, when a blade raises about its flap axis, theblade pitch is reduced by the delta 3 pitch-flap coupling, which isknown as positive delta 3 (flap up/pitch down). To achieve desiredstability for a conventional tiltrotor aircraft, however, when a bladeraises about its flap axis, the blade pitch is increased by the delta 3pitch-flap coupling, which is known as negative delta 3 (flap up/pitchup).

During high speed airplane mode flight, it is important to controlproprotor blade flapping on a tiltrotor aircraft, as the forwardairspeed may induce proprotor aeroelastic instability, similar topropeller whirl flutter, that may couple to the wing and lead tofailures. In addition, it can be important to maintain the flappingfrequency sufficiently distant from the first-in-plane frequency. Toachieve this balance, conventional tiltrotor aircraft have utilized anegative delta 3 angle of 15 degrees. Due to the location requirementsfor the pitch links and pitch horns necessary to achieve the negative 15degrees delta 3 configuration, proprotor systems have been limited tothe conventional three blade design. It is noted that for reasonsincluding pilot fatigue, passenger comfort, noise reduction andvibration induced mechanical failures, to name a few, it is desirable tohave more than three proprotor blades on each proprotor system of atiltrotor aircraft.

In the illustrated embodiment, each proprotor system 30 includes fiveproprotor blades 32 that are positioned circumferentially about a hub atapproximately seventy-two degree intervals. Preferably, proprotor blades32 are formed from a high-strength and lightweight material. Forexample, the structural components of proprotor blades 32 may be formedfrom carbon-based materials such as graphite-based materials,graphene-based materials or other carbon allotropes including carbonnanostructure-based materials such as materials including single-walledand multi-walled carbon nanotubes. In one example, the spar and/or skinof proprotor blades 32 are preferably monolithic structures formed usinga broad goods and/or layered tape construction process having a manualor automated layup of a plurality of composite broad goods materiallayers including carbon fabrics, carbon tapes and combinations thereof,positioned over one or more mandrels having simple geometric surfaceswith smooth transitions. After curing and other processing steps, thematerial layers form high-strength, lightweight solid composite members.In this process, the material thicknesses of the components can betailoring spanwise and chordwise to achieve the desired properties. Theproprotor blade components may be composed of up to about 50 percent,about 60 percent, about 70 percent, about 80 percent, about 90 percentor more of the carbon-based material or materials.

Proprotor blades 32 are preferably designed to a desired stiffnessand/or stiffness to mass ratio such that when operated within theproprotor systems of the present disclosure, the first-in-planefrequency of proprotor blades 32 is below 1.0/rev. For example, thefirst in-plane frequency of proprotor blades 32 may be between about0.6/rev and about 0.9/rev. In this example, the first in-plane frequencyof proprotor blades 32 in the helicopter mode of tiltrotor aircraft 10may be between about 0.6/rev and about 0.7/rev and the first in-planefrequency of proprotor blades 32 in the airplane mode of tiltrotoraircraft 10 may be between about 0.8/rev and about 0.9/rev. Maintainingthe first-in-plane frequency below 1.0/rev decouples the first-in-planelead-lag frequency from the per revolution excitations frequencies andthe out-of-plane flapping frequency. This decoupling allows a shift fromthe conventional negative 15 degrees delta 3 configuration to a positivedelta 3 configuration including up to about a positive 35 degrees delta3 configuration. Using the disclosed positive delta 3 configuration, thelocation requirements of the pitch links and pitch horns no longer limitthe proprotor design to the conventional three blade configuration andenable the five blade configurations of embodiments herein. In otherembodiments, a negative delta 3 can be used with the proprotor systemsof the present disclosure.

Referring next to FIGS. 2A-2D in the drawings, a soft-in-plane proprotorsystem for tiltrotor aircraft is depicted and generally designated 100.In the illustrated embodiment, proprotor system 100 includes a hub 102having five proprotor blades 104 coupled thereto at respective gripmembers depicted as devises 106 of hub 102. Hub 102 is attached to androtates with mast 108, which is coupled to a drive system including anengine and transmission of the tiltrotor aircraft that provides torqueand rotational energy to proprotor system 100 to enable rotation aboutrotational axis 110. In the illustrated embodiment, each proprotor blade104 includes an outer skin 112 having a spar 114 that extends spanwisetoward the tip of proprotor blade 104. Spars 114 are preferably the mainstructural member of proprotor blades 104 designed to carry the primarycentrifugal and bending loads of proprotor blades 104. Proprotor blades104 may have a root-to-tip twist on the order of about 20 degrees toabout 40 degrees or other suitable root-to-tip twist.

Each spar 114 has a root section depicted as integral cuff 116 to enablecoupling of each proprotor blade 104 with a respective bearing assembly118 disposed within a loop yoke 120. As best seen in FIG. 3, eachbearing assembly 118 includes a flapping bearing 122 coupled to a clevis106 of hub 102 by a connecting member depicted as pin 124. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including moving and/or non-moving mechanical connections. Asillustrated, flapping bearing 122 is a twin conical elastomeric flappingbearing operable to allow a proprotor blade 104 to rotate or have aflapping degree of freedom relative to hub 102 about a flapping axis 126that passes through pin 124. Flapping bearing 122 may be formed from oneor more elastomeric members or layers and may include rigid shimsdisposed between elastomeric layers. The durometer and thickness of thematerials as well as the stiffness of flapping bearing 122 may betailored to achieve the desired operational modes based upon the loadsand motions expected in the particular application.

Each bearing assembly 118 also includes a lead-lag damper 128 coupled toa clevis 106 of hub 102 by pin 124. As illustrated, lead-lag damper 128is an elastomeric damper having a spring rate operable to apply adamping force to the lead-lag degree of freedom of a proprotor blade 104and to tune the first-in-plane lead-lag frequency of in-planeoscillation of a proprotor blade 104. Lead-lag damper 128 may include aplurality of rigid shims disposed between layers of the elastomericmaterial. The durometer and thickness of the materials as well as thesoftness and/or a spring rate of lead-lag damper 128 may be tailored toachieve the desired operational modes based upon the loads and motionsexpected in the particular application.

In addition, each bearing assembly 118 includes a blade anchor 130 thathas a plurality of pins 132 for connection to a spar 114 of a proprotorblade 104. In the illustrated embodiment, the in-plane oscillation of aproprotor blade 104 is coupled to lead-lag damper 128 by blade anchor130 via blade anchor extension 134 that is coupled to lead-lag damper128. Blade anchor 130 has a bearing support 136 proximate its outboardend.

Each bearing assembly 118 further includes a centrifugal force bearing138. As illustrated, centrifugal force bearing 138 is a twin sphericalelastomeric bearing having a pair of oppositely disposed sphericalsurfaces, the first of which corresponds to a spherical surface ofbearing support 136 of blade anchor 130 and the second of whichcorresponds to a spherical surface of bearing support 140 that issecurably coupled to an outboard portion of loop yoke 120. Theconnections between centrifugal force bearing 138 and loop yoke 120 atbearing support 140 and between centrifugal force bearing 138 and bladeanchor 130 at bearing support 136 are permanent and may be made byvulcanizing the elastomeric material of centrifugal force bearing 138directly on these surfaces or by bonded, adhered or otherwise securedthe elastomeric material in a non-removable manner to these surfaces. Assuch, the spherical surfaces of bearing support 136 and bearing support140 along with centrifugal force bearing 138 may be considered a singlemechanical element. Centrifugal force bearing 138 may include aplurality of rigid shims disposed between layers of the elastomericmaterial. The durometer and thickness of the materials as well as thestiffness and/or spring rate of centrifugal force bearing 138 may betailored to achieve the desired operational modes based upon the loadsand motions expected in the particular application. In operation,centrifugal force bearing 138 is operable to provide a centrifugal forceretention load path from a proprotor blade 104 to hub 102 via loop yoke120 and pin 124. More specifically, loop yoke 120 includes a flappingbearing receiving region 144 and centrifugal force bearing receivingregion 146 that transfer centrifugal force from a proprotor blade 104 tohub 102.

As illustrated, each spar 114 is coupled to a respective bearingassembly 118 by pins 132 of blade anchor 130. Thus, each spar 114 has acentrifugal force retention load path through integral cuff 116 viabearing assembly 118 and loop yoke 120 to hub 102. As noted, eachproprotor blade 104 is operable to independently pivot or flap relativeto hub 102 about its respective flapping axis 126. In the illustratedembodiment, each spar 114 includes an integral pitch horn 148 that iscoupled to a pitch link 150 of a pitch control assembly 152 depicted asthe rotating portion of a rise and fall swash plate operable tocollectively and cyclically control the pitch of proprotor blades 104.Each proprotor blade 104 is operable to independently rotate about itspitch change axis 154 relative to hub 102, thereby changing pitchresponsive to changes in position of the respective pitch link 150.Rotation of each proprotor blade 104 causes the respective blade anchor130 to rotate relative to the lead-lag damper 128. As best seen in FIG.2A, angle 156 represents the positive delta 3 configuration of thepresent embodiment, wherein the delta 3 angle is about positive 35degrees. Implementing the illustrated positive delta 3 configurationenables the five blade design of proprotor system 100 while avoidinginterference between pitch links 150 and other components of proprotorsystem 100.

Referring next to FIGS. 4A-4C in the drawings, the operation of anelastomeric lead-lag damper is depicted during in-plane oscillation of aproprotor blade. In the illustrated embodiment, spar 114 of proprotorblade 104 is coupled to hub 102 by loop yoke 120 and bearing assembly108 including flapping bearing 122, lead-lag damper 128, blade anchor130 and centrifugal force bearing 138. During operation of a proprotorsystem, the proprotor blades may tend to oscillate forward (see leadposition in FIG. 4A) and backwards (see lag position in FIG. 4C)relative to a neutral position (see FIG. 4B) as the proprotor systemrotates as a result of conservation of momentum andacceleration/deceleration caused by the Coriolis effect. As illustrated,lead-lag damper 128 is an elastomeric damper having a spring rateoperable to apply a damping force to prevent excess back and forthmovement of proprotor blade 104 and to tune the first-in-plane lead-lagfrequency a proprotor blade 104 to be below 1/rev through materialselection, component sizing, component design and other factors known tothose having ordinary skill in the art. For example, the first in-planefrequency of proprotor blade 104 may be between about 0.6/rev and about0.9/rev. In this example, the first in-plane frequency of proprotorblade 104 in the helicopter mode of a tiltrotor aircraft may be betweenabout 0.6/rev and about 0.7/rev and the first in-plane frequency ofproprotor blade 104 in the airplane mode of a tiltrotor aircraft may bebetween about 0.8/rev and about 0.9/rev. Maintaining the first-in-planefrequency below 1.0/rev decouples the first-in-plane lead-lag frequencyfrom the per revolution excitations frequencies and the out-of-planeflapping frequency.

In the illustrated embodiment, proprotor blade 104 has a virtuallead-lag hinge disposed within loop yoke 120 depicted as lead-lag axis156, which is normal to pitch change axis 154, pointing out of the page,and coincident with a center point of centrifugal force bearing 138, asbest seen in FIG. 4B. In other embodiments, it should be noted by thosehave ordinary skill in the art that the virtual lead-lag hinge could beinboard or outboard of the location of virtual lead-lag hinge 156. Asbest seen in FIG. 4A, when proprotor blade 104 moves forward in the leadposition, proprotor blade 104 pivots about lead-lag axis 156 such thatblade anchor extension 134 moves backwards causing spindle 158 of bladeanchor 130 to shift piston 160 of lead-lag damper 128 backwards. Thebackwards movement of piston 160 is countered by the elastomer oflead-lag damper 128. Likewise, as best seen in FIG. 4C, when proprotorblade 104 moves backwards in the lag position, proprotor blade 104pivots about lead-lag axis 156 such that blade anchor extension 134moves forward causing spindle 158 of blade anchor 130 to shift piston160 of lead-lag damper 128 forward. The forward movement of piston 160is also countered by the elastomer of lead-lag damper 128. In thismanner, lead-lag damper 128 applies a damping force to piston 160 andthus to proprotor blade 104 to prevent excess back and forth movementand to tune the first-in-plane lead-lag frequency of proprotor blade 104to be below 1/rev.

Referring next to FIGS. 5A-5C in the drawings, the operation of a fluidlead-lag damper is depicted during in-plane oscillation of a proprotorblade. In the illustrated embodiment, spar 114 of proprotor blade 104 iscoupled to hub 102 by loop yoke 120 and bearing assembly 170 includingflapping bearing 122, lead-lag damper 172, blade anchor 130 andcentrifugal force bearing 138. During operation of a proprotor system,the proprotor blades may tend to oscillate forward (see lead position inFIG. 5A) and backwards (see lag position in FIG. 5C) relative to aneutral position (see FIG. 5B) as the proprotor system rotates as aresult of conservation of momentum and acceleration/deceleration causedby the Coriolis effect. As illustrated, lead-lag damper 172 is a fluiddamper having a spring rate operable to apply a damping force to preventexcess back and forth movement of proprotor blade 104 and to tune thefirst-in-plane lead-lag frequency a proprotor blade 104 to be below1/rev through fluid selection, component sizing, component design andother factors known to those having ordinary skill in the art. Forexample, the first in-plane frequency of proprotor blade 104 may bebetween about 0.6/rev and about 0.9/rev. In this example, the firstin-plane frequency of proprotor blade 104 in the helicopter mode of atiltrotor aircraft may be between about 0.6/rev and about 0.7/rev andthe first in-plane frequency of proprotor blade 104 in the airplane modeof a tiltrotor aircraft may be between about 0.8/rev and about 0.9/rev.Maintaining the first-in-plane frequency below 1.0/rev decouples thefirst-in-plane lead-lag frequency from the per revolution excitationsfrequencies and the out-of-plane flapping frequency.

In the illustrated embodiment, proprotor blade 104 has a virtuallead-lag hinge disposed within loop yoke 120 depicted as lead-lag axis156, which is normal to pitch change axis 154, pointing out of the page,and coincident with a center point of centrifugal force bearing 138, asbest seen in FIG. 5B. In other embodiments, it should be noted by thosehave ordinary skill in the art that the virtual lead-lag hinge could beinboard or outboard of the location of virtual lead-lag hinge 156. Asbest seen in FIG. 5A, when proprotor blade 104 moves forward in the leadposition, proprotor blade 104 pivots about lead-lag axis 156 such thatblade anchor extension 134 moves backwards causing spindle 158 of bladeanchor 130 to shift piston 174 of lead-lag damper 172 backwards. Thebackwards movement of piston 174 is countered by fluid resistance asfluid transfers between chambers 176, 178 through passageways 180, 182.Likewise, as best seen in FIG. 5C, when proprotor blade 104 movesbackwards in the lag position, proprotor blade 104 pivots about lead-lagaxis 156 such that blade anchor extension 134 moves forward causingspindle 158 of blade anchor 130 to shift piston 174 of lead-lag damper172 forward. The forward movement of piston 174 is countered by fluidresistance as fluid transfers between chambers 176, 178 throughpassageways 180, 182. In this manner, lead-lag damper 172 applies adamping force to piston 174 and thus to proprotor blade 104 to preventexcess back and forth movement and to tune the first-in-plane lead-lagfrequency of proprotor blade 104 to be below 1/rev.

Referring next FIGS. 6A-6C of the drawings, various views of a loop yoke120 are provided. As discussed herein, each loop yoke 120 provides acentrifugal force retention load path between a proprotor blade 104 andhub 102 of a proprotor system such as soft-in-plane proprotor system 100operable for use on tiltrotor aircraft. In the illustrated embodiment,loop yoke 120 is a high-strength, lightweight, solid composite memberhaving a profile in the form of a continuous loop 200, a best seen inthe cross sectional view of FIG. 6C. As best seen in the perspectiveview of FIG. 6A, loop yoke 120 has a longitudinal axis 202. In theillustrated embodiment, continuous loop 200 includes an upperlongitudinal section 204 and a lower longitudinal section 206. Inaddition, continuous loop 200 includes an inboard arcuate section 208and an outboard arcuate section 210. Upper longitudinal section 204 andlower longitudinal section 206 respectively extend between inboardarcuate section 208 and outboard arcuate section 210 to form continuousloop 200.

Loop yoke 120 includes a flapping bearing receiving region 212 disposedat least partially within inboard arcuate section 208 to the interior ofcontinuous loop 200. Loop yoke 120 also includes a centrifugal forcebearing receiving region 214 disposed at least partially within outboardarcuate section 210 to the interior of continuous loop 200. In theillustrated embodiment, upper longitudinal section 204 and lowerlongitudinal section 206 are generally parallel to one another withupper longitudinal section 204 having a tapered section 216 and agenerally constant width section 218 and lower longitudinal section 206having a tapered section 220 and a generally constant width section 222such that inboard arcuate section 208 has a greater width than outboardarcuate section 210. Even though a particular design for loop yoke 120has been depicted and described, it should understood by those havingordinary skill in the art that yoke loops of the present disclosurecould have alternate configurations including loop yokes having upperand lower longitudinal sections that are not parallel to one another,loop yokes having upper and lower longitudinal sections that are fullytapered, not tapered or have other contours and loop yokes havinginboard and outboard arcuate sections having the same width, to name afew.

Referring additionally to FIGS. 7A-7E of the drawings, therein isdepicted a process of forming loop yokes of the present disclosureaccording to one example embodiment. FIG. 7A shows a layup processincluding steps 250-264 corresponding to the layup of variously plies ofmaterial. As indicated by the ellipses between various steps, theteachings herein recognize that many more plies would be used in anactual layup to achieved the desired wall thickness for the loop yokesof the present disclosure. For example, depending upon the material ormaterials forming the plies and the desired wall thickness of the loopyokes being formed, dozens or hundreds of plies may be used.

At step 250, a ply 266 of material has been wrapped around mandrel 248for substantially one turn including allowance for overlapping and/orgapping between the ends of ply 266. Mandrel 248 has a smother outersurface corresponding to the desired shape of the interior of the loopyokes. In the illustrated embodiment, ply 266 is a double bias materialply such as a double bias material ply having plus and minus 45 degreeorientation. It is noted that only the outer portion of the double biasmaterial ply is visible in FIG. 7A, which is being represented by theplus 45 degree parallel lines depicting ply 266. As one example, ply 266may be a double bias carbon fiber fabric with plus and minus 45 degreeorientation having a thickness of between about 10 and 20 thousandth ofan inch. It should be understood by those having ordinary skill in theart that the loop yokes of the present disclosure could be formed usingother types of broad goods as the innermost ply including fabrics otherthan carbon fabrics, construction other than plus and minus 45 degreeorientation and thicknesses both less than 10 thousandth of an inch andgreater than 20 thousandth of an inch including, for example, broadgoods having plain weaves, twill weaves or unidirectional constructionand broad goods formed from fiberglass, to name a few.

At step 252, ply 268 has been wrapped around mandrel 248 to the exteriorof ply 266 for substantially one turn. In the illustrated embodiment,ply 268 is a unidirectional material ply such as a unidirectionalmaterial ply with 0 degree orientation parallel to the longitudinal axisof the loop yoke, as represented by the 0 degree parallel linesdepicting ply 268. As one example, ply 268 may be unidirectional carbonfiber fabric with 0 degree orientation having a thickness of betweenabout 5 and 15 thousandth of an inch. It should be understood by thosehaving ordinary skill in the art that the loop yokes of the presentdisclosure could be formed using other types of broad goods for theintermediate plies including fabrics other than carbon fabrics,construction other than 0 degree orientation and thicknesses both lessthan 5 thousandth of an inch and greater than 15 thousandth of an inchincluding, for example, broad goods having plain weaves or twill weavesand broad goods formed from fiberglass, to name a few.

Preferably, the same broad goods material may be wrapped around mandrel248 more than one time during certain stages of the layup process. Forexample, at step 254, ply 270 has been wrapped around mandrel 248 to theexterior of ply 268 for substantially one turn. In this configuration,two plies of the unidirectional material have been wrapped over one plyof the double bias material, which would result in a ratio ofunidirectional material plies to the double bias material plies of about2 to 1. As stated above, the ellipsis between step 252 and step 254represents additional turns of the broad goods material being wrappedaround mandrel 248. For example, the unidirectional material may bewrapped around mandrel 248 any number of times depending upon thedesired component properties. For example, the unidirectional materialmay be wrapped around mandrel 248 one, two, three, four or moreadditional turns. In a preferred arrangement, the ratio of theunidirectional material plies to the double bias material plies isbetween about 2 to 1 and about 6 to 1. In another preferred arrangement,the ratio of the unidirectional material plies to the double biasmaterial plies is between about 3 to 1 and about 5 to 1. In a furtherpreferred arrangement, the ratio of the unidirectional material plies tothe double bias material plies is about 4 to 1. It is noted thatorienting the unidirectional material plies with 0 degree orientationparallel to the longitudinal axis of the loop yokes provides thegreatest strength in the direction of the primary load carried by theloop yokes; namely, the centrifugal force load path supported by theloop yokes.

Continuing with the layup process, at step 256, ply 272 has been wrappedaround mandrel 248 to the exterior of ply 270 for substantially oneturn. In the illustrated embodiment, ply 272 is a double bias materialply such as a double bias carbon fiber fabric having plus and minus 45degree orientation. At step 258, ply 274 has been wrapped around mandrel248 to the exterior of ply 272 for substantially one turn. In theillustrated embodiment, ply 274 is a unidirectional material ply such asa unidirectional carbon fiber fabric with 0 degree orientation parallelto the longitudinal axis of the loop yoke. Likewise, at step 260, ply276 has been wrapped around mandrel 248 to the exterior of ply 274 forsubstantially one turn. In the illustrated embodiment, ply 276 is aunidirectional material ply such as a unidirectional carbon fiber fabricwith 0 degree orientation parallel to the longitudinal axis of the loopyoke. As stated above, the ellipsis between step 258 and step 260represent any number of additional turns of the broad goods materialbeing wrapped around mandrel 248. At step 262, ply 278 has been wrappedaround mandrel 248 to the exterior of ply 276 for substantially oneturn. In the illustrated embodiment, ply 276 is a double bias materialply such as a double bias carbon fiber fabric having plus and minus 45degree orientation. It is noted that having a double bias material plyinterposed between multiple unidirectional material plies improves theperformance of the loop yokes by preventing any crack that may developwithin one group of unidirectional material plies from propagating toanother group of unidirectional material plies.

The layup process continues by sequencing between laying up a pluralityof unidirectional material plies then laying up a double bias materialply until the desired thickness for the loop yokes is achieved, asrepresented by the ellipsis between step 262 and step 264. At step 264,ply 280 has been wrapped around mandrel 248 to form the outermostmaterial ply. In the illustrated embodiment, ply 280 is a double biasmaterial ply such as a double bias carbon fiber fabric having plus andminus 45 degree orientation. It is noted that the ratio of a first typeof material plies, such as unidirectional material plies, to a secondtype of material plies, such as double bias material plies, need notstay constant for the entire thickness of the loop yokes. For example,in certain implementations, it may be desirable to have one ratio of thefirst type of material plies to the second type of material plies, suchas between about 2 to 1 and about 3 to 1, in the inner and/or outerportions of the loop yoke but have a second ratio of the first type ofmaterial plies to the second type of material plies, such as betweenabout 4 to 1 and about 6 to 1, in the center sections of the loop yoke.Having nonuniform ratios of the first type of material plies to thesecond type of material plies enables tailoring of desired properties ofthe loop yoke. Also, even though the double bias material plies havebeen described as being prefabricated as a single fabric, it should beunderstood by those having ordinary skill in the art that the doublebias material plies could alternatively be formed during lay up by, forexample, wrapping a unidirectional material ply such as a unidirectionalcarbon fiber fabric with plus 45 degree orientation to the exterior of aunidirectional material ply such as a unidirectional carbon fiber fabricwith minus 45 degree orientation. In addition, even though the doublebias material plies have been described as being plus and minus 45degrees, it should be understood by those having ordinary skill in theart that the double bias material plies could alternatively have otherorientations such as plus and minus 30 degrees, plus and minus 60degrees, plus 30 degrees and minus 60 degrees or other suitableorientation depending upon the desired properties for the loop yokes.

The layup process for the loop yokes of the present disclosure may be amanual process or an automated process. The material plies may be laidup with a fluid resin such as an epoxy resin. The combination of thematerial plies and resin, supported by mandrel 248, may be cured using,for example, an autoclave curing process, as indicated by the heatarrows in FIG. 7B. In the illustrated embodiment, the curing processyields a composite structure in the form a cured yoke assembly.Preferably, the cured yoke assembly is cut into a plurality of yokemembers as depicted in FIG. 7C. The yoke members are then trimmed usinga suitable machining or other removal process to form the yoke membersinto the desired shape of the loop yokes of the present disclosure asdepicted in FIG. 7D. Various additional finishing steps may then beperformed to produced the loop yokes of the present disclosure asdepicted in FIG. 7E. Through the use of the broad goods manufacturingprocess and using carbon fiber fabric as the primary structuralmaterial, the loop yokes of the present disclosure are high-strength,lightweight, solid composite members operable to provides a centrifugalforce retention load path between proprotor blades 104 and hub 102 of aproprotor system such as soft-in-plane proprotor system 100 operable foruse on tiltrotor aircraft. In addition, the loop yokes of the presentdisclosure have suitable fatigue durability for their intended purpose.

Referring next to FIGS. 8A-8D in the drawings, a soft-in-plane proprotorsystem for tiltrotor aircraft is depicted and generally designated 300.In the illustrated embodiment, proprotor system 300 includes a hub 302having five proprotor blades 304 coupled thereto at respective gripmembers depicted as devises 306 of hub 302. Hub 302 is attached to androtates with mast 308, which is coupled to a drive system including anengine and transmission of the tiltrotor aircraft that provides torqueand rotational energy to proprotor system 300 to enable rotation aboutrotational axis 310. In the illustrated embodiment, each proprotor blade304 includes an outer skin 312 having a spar 314 that extends spanwisetoward the tip thereof. Spars 314 are preferably the main structuralmember of proprotor blades 304 designed to carry the primary centrifugaland bending loads of proprotor blades 304. Proprotor blades 304 may havea root-to-tip twist on the order of about 20 degrees to about 40 degreesor other suitable root-to-tip twist.

Each spar 314 has a root section depicted as integral cuff 316 to enablecoupling of each proprotor blade 304 with a respective blade supportassembly 318. As best seen in FIG. 9, each blade support assembly 318includes a flapping bearing 322 coupled to a clevis 306 of hub 302 by aconnecting member depicted as pin 324. As illustrated, flapping bearing322 is a twin conical elastomeric flapping bearing operable to allow aproprotor blade 304 to rotate or have a flapping degree of freedomrelative to hub 302 about a flapping axis 326 that passes through pin324. Flapping bearing 322 may be formed from one or more elastomericmembers or layers and may include rigid shims disposed betweenelastomeric layers. The durometer and thickness of the materials as wellas the stiffness of flapping bearing 322 may be tailored to achieve thedesired operational modes based upon the loads and motions expected inthe particular application.

Each blade support assembly 318 also includes a lead-lag damper 328coupled to a clevis 306 of hub 302 by pin 324. As illustrated, lead-lagdamper 328 is an elastomeric damper having a spring rate operable toapply a damping force to the lead-lag degree of freedom of a proprotorblade 304 and to tune the first-in-plane lead-lag frequency of in-planeoscillation of a proprotor blade 304. Lead-lag damper 328 may include aplurality of rigid shims disposed between layers of the elastomericmaterial. The durometer and thickness of the materials as well as thesoftness and/or a spring rate of lead-lag damper 328 may be tailored toachieve the desired operational modes based upon the loads and motionsexpected in the particular application.

In addition, each blade support assembly 318 includes a blade anchor 330that has a plurality of pins 332 for connection to a spar 314 of aproprotor blade 304. In the illustrated embodiment, the in-planeoscillation of a proprotor blade 304 is coupled to lead-lag damper 328by blade anchor 330 via blade anchor extension 334 that is coupled tolead-lag damper 328. The connection between blade anchor 330 andproprotor blade 304 is at an inboard station of proprotor blade 304.

Each blade support assembly 318 further includes a twist shank 338 thatis operable to provide a centrifugal force retention load path from aproprotor blade 304 to hub 302 via yoke 320 and pin 324. Morespecifically, yoke 320 includes a flapping bearing receiving region 344and grips 346 that couple to an inboard end 372 of twist shank 338 viaconnecting members depicted as pins 360. In the illustrated embodiment,upper and lower spacers 362, 364 are disposed between yoke 320 and twistshank 338. The outboard end 376 of twist shank 338 is coupled toproprotor blade 304 via connecting members depicted as pins 366. Theconnection between twist shank 338 and proprotor blade 304 is at anoutboard station of proprotor blade 304. Preferably, twist shank 338 isstiff in the spanwise direction but flexible in the in-plain andtorsional degrees of freedom.

As best seen in FIGS. 10A-10C, twist shank 338 is formed from aplurality of material layers such as alternating high strength layershaving resilient layers interposed therebetween. For example, the highstrength layers may be formed from fiberglass or carbon while theresilient layers may be formed from rubber or other elastomer orpolymer. Depending upon the desired modes of operations for twist shank338, the number of high strength layers may be between three and fifteenor more layers. In some implementations, the material layers of twistshank 338 may be cured, bonded or otherwise adhered together, in whichcase, bending and torsional stress applied to twist shank 338 results inshear deformation of the resilient layers. In other implementations,some or all of the material layers along all or a portion of twist shank338 may be free to move relative to each other to allow for bending andtorsion of twist shank 338. In either case, twist shank 338 preferablyhas an in-plane spring rate operable to assist lead-lag damper 328 inapplying a damping force to the lead-lag degree of freedom of aproprotor blade 304 and to tune the first-in-plane lead-lag frequency ofin-plane oscillation of a proprotor blade 304.

As illustrated, twist shank 338 is in the form of a beam 370 having arectangular cross section (as seen in FIG. 10C), an inboard end 372, acentral section 374 and an outboard end 376. Twist shank 338 has atapered section 378 between inboard end 372 and central section 374. Inaddition, twist shank 338 has a tapered section 380 between outboard end376 and central section 374. Preferably, the design and materials oftwist shank 338 enable the attached proprotor blade 304 to rotate aboutits pitch change axis 354 through a collective range between about plus50 degrees and about minus 50 degrees with inboard end 372 remainingfixed while outboard end 376 rotates relative thereto with proprotorblade 304. Twist shank 338 may have a pre-twist in its resting state incertain implementations. For example, twist shank 338 may have betweenabout plus or minus 5 degrees to 20 degrees of pre-twist.

Returning to FIGS. 8A-8D, each proprotor blade 304 has a primarycentrifugal force retention load path through twist shank 338 and yoke320 to hub 102 via pins 366. Each spar 314 is also coupled to arespective blade support assembly 318 by pins 332 of blade anchor 330.As noted, each proprotor blade 304 is operable to independently pivot orflap relative to hub 302 about its respective flapping axis 326. In theillustrated embodiment, each spar 314 includes an integral pitch horn348 that is coupled to a pitch link 350 of a pitch control assembly 352depicted as the rotating portion of a rise and fall swash plate operableto collectively and cyclically control the pitch of proprotor blades304. Each proprotor blade 304 is operable to independently rotate aboutits pitch change axis 354 relative to hub 302, thereby changing pitchresponsive to changes in position of the respective pitch link 350.Pitch changes of each proprotor blade 304 cause the respective bladeanchor 330 to rotate relative to the lead-lag damper 328 and causetorsional deformation of twist shank 338. As best seen in FIG. 8A, angle356 represents the positive delta 3 configuration of the presentembodiment, wherein the delta 3 angle is about positive 35 degrees.Implementing the illustrated positive delta 3 configuration enables thefive blade design of proprotor system 300 while avoiding interferencebetween pitch links 350 and other components of proprotor system 300.

Referring additionally to FIGS. 11A-11C in the drawings, the operationof an elastomeric lead-lag damper and a twist shank is depicted duringin-plane oscillation of a proprotor blade. In the illustratedembodiment, proprotor blade 304 is coupled to blade support assembly 318at pins 332 and pins 366. Blade support assembly 318 including yoke 320,flapping bearing 322, lead-lag damper 328, blade anchor 330 and twistshank 338 are coupled to hub 302 at pin 324. During operation of aproprotor system, the proprotor blades may tend to oscillate forward(see lead position in FIG. 11A) and backwards (see lag position in FIG.11C) relative to a neutral position (see FIG. 11B) as the proprotorsystem rotates as a result of conservation of momentum andacceleration/deceleration caused by the Coriolis effect. As illustrated,lead-lag damper 328 has an in-plane spring rate and twist shank 338 hasan in-plane spring rate that together apply a damping force to preventexcess back and forth movement of proprotor blade 304 and tune thefirst-in-plane lead-lag frequency of proprotor blade 304 to be below1/rev through material selection, component sizing, component design andother factors known to those having ordinary skill in the art. Forexample, the first in-plane frequency of proprotor blade 304 may bebetween about 0.6/rev and about 0.9/rev. In this example, the firstin-plane frequency of proprotor blade 304 in the helicopter mode of atiltrotor aircraft may be between about 0.6/rev and about 0.7/rev andthe first in-plane frequency of proprotor blade 304 in the airplane modeof a tiltrotor aircraft may be between about 0.8/rev and about 0.9/rev.Maintaining the first-in-plane frequency below 1.0/rev decouples thefirst-in-plane lead-lag frequency from the per revolution excitationsfrequencies and the out-of-plane flapping frequency.

In the illustrated embodiment, proprotor blade 304 has a virtuallead-lag hinge disposed along twist shank 338 depicted as lead-lag axis356, which is normal to pitch change axis 354, pointing out of the page,as best seen in FIG. 11B. Preferably, virtual lead-lag hinge 356 isdisposed between about a 15 percent station and about a 40 percentstation of twist shank 338 measured from the inboard end of twist shank338 toward the outboard end of twist shank 338. In one example, virtuallead-lag hinge 356 is disposed between about a 20 percent station andabout a 30 percent station of twist shank 338. As best seen in FIG. 11A,when proprotor blade 304 moves forward in the lead position, proprotorblade 304 pivots about lead-lag axis 356 such that blade anchorextension 334 moves backwards causing spindle 358 of blade anchor 330 toshift piston 390 of lead-lag damper 328 backwards. The backwardsmovement of piston 390 is countered by the elastomer of lead-lag damper328. In addition, when proprotor blade 304 moves forward in the leadposition, twist shank 338 bends such that outboard end 376 also movesforward with proprotor blade 304 while inboard end 372 remainssubstantially fixed. The forward movement of outboard end 376 of twistshank 338 is countered by the damping force of twist shank 338.Likewise, as best seen in FIG. 11C, when proprotor blade 304 movesbackwards in the lag position, proprotor blade 304 pivots about lead-lagaxis 356 such that blade anchor extension 334 moves forward causingspindle 358 of blade anchor 330 to shift piston 390 of lead-lag damper328 forward. The forward movement of piston 390 is also countered by theelastomer of lead-lag damper 328. In addition, when proprotor blade 304moves backwards in the lag position, twist shank 338 bends such thatoutboard end 376 also moves backwards with proprotor blade 304 whileinboard end 372 remains substantially fixed. The backwards movement ofoutboard end 376 of twist shank 338 is countered by the damping force oftwist shank 338. In this manner, lead-lag damper 328 and twist shank 338apply a damping force to proprotor blade 304 to prevent excess back andforth movement and to tune the first-in-plane lead-lag frequency ofproprotor blade 304 to be below 1/rev.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A soft-in-plane proprotor system operable for useon a tiltrotor aircraft, the proprotor system comprising: a hub; aplurality of proprotor blades coupled to the hub such that eachproprotor blade is operable to independently flap relative to the hubabout a flapping axis, independently change pitch about a pitch changeaxis and independently oscillate in-plane about a lead-lag axis; aplurality of loop yokes, each coupling one of the proprotor blades withthe hub and each including: a continuous loop having first and secondlongitudinal sections extending between inboard and outboard arcuatesections; a flapping bearing receiving region disposed at leastpartially within the inboard arcuate section to an interior of thecontinuous loop; and a centrifugal force bearing receiving regiondisposed at least partially within the outboard arcuate section to theinterior of the continuous loop; and a plurality of bearing assemblies,each disposed within one of the continuous loops and each including: aflapping bearing received by the flapping bearing receiving region ofthe respective continuous loop and coupled to the hub; a lead-lag dampercoupled to the hub; a centrifugal force bearing received by thecentrifugal force bearing receiving region of the respective continuousloop; and a blade anchor positioned between the lead-lag damper and thecentrifugal force bearing, the blade anchor coupled to the respectiveproprotor blade between the lead-lag damper and the centrifugal forcebearing such that the respective lead-lag axis is disposed within therespective proprotor blade; wherein, each of the continuous loopscomprises a composite material having a plurality of double biasmaterial plies and a plurality of unidirectional material plies; andwherein, the number of unidirectional material plies is greater than thenumber of double bias material plies.
 2. The proprotor system as recitedin claim 1 wherein the composite material of each of the continuousloops further comprises at least two of the unidirectional materialplies interposed between the double bias material plies.
 3. Theproprotor system as recited in claim 1 wherein the composite material ofeach of the continuous loops further comprises a ratio of unidirectionalmaterial plies to double bias material plies between about 2 to 1 andabout 6 to
 1. 4. The proprotor system as recited in claim 1 wherein thecomposite material of each of the continuous loops further comprises aratio of unidirectional material plies to double bias material pliesbetween about 3 to 1 and about 5 to
 1. 5. The proprotor system asrecited in claim 1 wherein the composite material of each of thecontinuous loops further comprises a ratio of unidirectional materialplies to double bias material plies of about 4 to
 1. 6. The proprotorsystem as recited in claim 1 wherein the double bias material plies ofthe composite material of each of the continuous loops further comprisedouble bias carbon fiber fabric.
 7. The proprotor system as recited inclaim 1 wherein the double bias material plies of the composite materialof each of the continuous loops further comprise double bias carbonfiber fabric with plus and minus 45 degree orientation relative to alongitudinal axis of the respective continuous loop.
 8. The proprotorsystem as recited in claim 1 wherein the unidirectional material pliesof the composite material of each of the continuous loops furthercomprise unidirectional carbon fiber fabric.
 9. The proprotor system asrecited in claim 1 wherein the unidirectional material plies of thecomposite material of each of the continuous loops further compriseunidirectional carbon fiber fabric with 0 degree orientation parallel toa longitudinal axis of the respective continuous loop.
 10. The proprotorsystem as recited in claim 1 wherein the first and second longitudinalsections of each of the continuous loops are generally parallel to oneanother.
 11. The proprotor system as recited in claim 1 wherein thefirst and second longitudinal sections of each of the continuous loopsare tapered between the inboard arcuate section and the outboard arcuatesection.
 12. A tiltrotor aircraft, comprising: a fuselage; a wingextending from the fuselage; at least one drive system supported by atleast one of the fuselage and the wing; and at least one soft-in-planeproprotor system coupled to the drive system, the proprotor systemincluding: a hub; a plurality of proprotor blades coupled to the hubsuch that each proprotor blade is operable to independently flaprelative to the hub about a flapping axis, independently change pitchabout a pitch change axis and independently oscillate in-plane about alead-lag axis; a plurality of loop yokes, each coupling one of theproprotor blades with the hub and each including: a continuous loophaving first and second longitudinal sections extending between inboardand outboard arcuate sections; a flapping bearing receiving regiondisposed at least partially within the inboard arcuate section to aninterior of the continuous loop; and a centrifugal force bearingreceiving region disposed at least partially within the outboard arcuatesection to the interior of the continuous loop; and a plurality ofbearing assemblies, each disposed within one of the continuous loops andeach including: a flapping bearing received by the flapping bearingreceiving region of the respective continuous loop and coupled to thehub; a lead-lag damper coupled to the hub; a centrifugal force bearingreceived by the centrifugal force bearing receiving region of therespective continuous loop; and a blade anchor positioned between thelead-lag damper and the centrifugal force bearing, the blade anchorcoupled to the respective proprotor blade between the lead-lag damperand the centrifugal force bearing such that the respective lead-lag axisis disposed within the respective proprotor blade; wherein, each of thecontinuous loops comprises a composite material having a plurality ofdouble bias material plies and a plurality of unidirectional materialplies; and wherein, the number of unidirectional material plies isgreater than the number of double bias material plies.
 13. The tiltrotoraircraft as recited in claim 12 wherein the composite material of eachof the continuous loops further comprises at least two of theunidirectional material plies interposed between the double biasmaterial plies.
 14. The tiltrotor aircraft as recited in claim 12wherein the composite material of each of the continuous loops furthercomprises a ratio of unidirectional material plies to double biasmaterial plies between about 2 to 1 and about 6 to
 1. 15. The tiltrotoraircraft as recited in claim 12 wherein the composite material of eachof the continuous loops further comprises a ratio of unidirectionalmaterial plies to double bias material plies between about 3 to 1 andabout 5 to
 1. 16. The tiltrotor aircraft as recited in claim 12 whereinthe composite material of each of the continuous loops further comprisesa ratio of unidirectional material plies to double bias material pliesof about 4 to
 1. 17. The tiltrotor aircraft as recited in claim 12wherein the double bias material plies of the composite material of eachof the continuous loops further comprise double bias carbon fiberfabric.
 18. The tiltrotor aircraft as recited in claim 12 wherein thedouble bias material plies of the composite material of each of thecontinuous loops further comprise double bias carbon fiber fabric withplus and minus 45 degree orientation relative to a longitudinal axis ofthe respective continuous loop.
 19. The tiltrotor aircraft as recited inclaim 12 wherein the unidirectional material plies of the compositematerial of each of the continuous loops further comprise unidirectionalcarbon fiber fabric.
 20. The tiltrotor aircraft as recited in claim 12wherein the unidirectional material plies of the composite material ofeach of the continuous loops further comprise unidirectional carbonfiber fabric with 0 degree orientation parallel to a longitudinal axisof the respective continuous loop.